The present invention relates to a linear freezer for cooling and/or chilling and/or crust freezing and/or freezing various products using gaseous and solid carbon dioxide, or gaseous and liquid nitrogen.
Such type of linear freezers are known from U.S. Pat. No. 3,841,109 and 3,879,954 incorporated herein as references.
They are composed of one or several adjacent modular sections forming an insulated enclosure through which various products are transported by one stainless steel endless conveyor belt (single tier freezer) or by three parallel and superposed stainless steel endless conveyor belts (triple tier freezer) of selected width from one entry opening (generally of manually adjustable cross-section) to one exit opening (generally of manually adjustable cross-section). Carbon dioxide gas atmosphere of said freezer is maintained at low temperatures by automatically regulated injection/expansion of high pressure liquid carbon dioxide into ambient-pressure carbon dioxide gas solid mixture through regularly spaced and orientable injection orifices in injection headers located in the vinicity of the roof of each modular section. (When nitrogen is used, nitrogen gas atmosphere is maintened at low temperature by automatically regulated injection of pressurized liquid nitrogen). Removal of heat from products occurs through forced gas convection created by two fans of given characteristics in each modular section and by kinetic energy of previously mentioned carbon dioxide expansion jets from injection orifices and through sublimation of carbon dioxide snow particles deposited on the surface of products or through evaporation of liquid nitrogen droplets on the surface of products. Said fans, of diameter equal to or less than the previously mentioned width of the conveyor belt, run at a high, manually selectable velocity and have a high delivery flowrate, thereby cresting a high heat removal capability and hence a high performance for said freezer.
These known linear freezers while satisfactory, thanks to their high performances, exhibit low efficiency because carbon dioxide or nitrogen is only partially used to remove heat from the products, the remaining portion being lost because of heat conduction through the freezer insulation, of degradation into heat of the kinetic energy of the flow created by the fans, of heat input from the product conveyor belt after warm-up in product unloading and especially product loading zones, and of heat input from large infiltrations of warm moist room air into the freezer atmosphere.
These known linear freezers which are sold by the CARDOX Corporation a division of LIQUID AIR Corporation under the commercial name "ULTRAFREEZE" do not have however the drawbacks exhibited by other freezers : because of the high velocity and high delivery of the fans used in said "ULTRAFREEZE" freezer, the fan blades do not cover with ice and frost and thereby do not loose their efficiency and delivered flow characteristics (flowrate, flow speed, flow distribution). Furthermore, they utilize the kinetic energy of carbon dioxide espansion jets and the sublimation latent heat of carbon dioxide snow particles or evaporation of nitrogen droplets in addition to the velocity of the flow created by the fans for heat removal from the products, thereby further enhancing the performances (i.e. capacity production) compared to those of said other freezers.